1. Field of the Invention
The present invention relates to a film formation apparatus for forming a structure on a substrate by spraying a law material powder at a high speed and deposit the powder thereon.
2. Description of a Related Art
Recent years, in the field of micro electrical mechanical system (MEMS), fabrication of sensors, actuators, or the like employing piezoelectric ceramic by film formation has been studied in order to further integrate those elements for practical use. As one of the film formation methods, the aerosol deposition (AD) method known as a technology for forming a film of ceramic, metal, etc. receives attention. The AD method is a film formation method of generating an aerosol containing a raw material powder and injecting it toward a substrate from a nozzle and depositing the raw material on the substrate. Here, the aerosol refers to solid or liquid fine particles floating in a gas.
In the AD method, the raw material powder accelerated at a high speed under a certain condition collides against an under layer such as the substrate or a previously formed deposition materials, etc. and cut into it, and, at the time of collision, the powder is crushed into particles of several tens of nanometers and new active surfaces appears, and then, film formation is performed by mechanochemical reaction in which the active surfaces firmly bind together. According to the AD method, a dense and strong thick film including no impurities can be formed. Accordingly, it is expected that a ceramic piezoelectric film to be used for piezoelectric actuators, piezoelectric pumps, inkjet printer heads, ultrasonic transducers, etc. is formed by the AD method, and thereby, the performance of those devices is improved. In addition, the AD method is also referred to as injection deposition method or gas deposition method.
In the AD method, it is not easy to fabricate a ceramic structure having a uniform film thickness and uniform film quality, and therefore, control of the film thickness and film quality becomes a problem. Since the film formation speed in the AD method vary delicately according to various conditions such as aerosol concentration, injection speed of aerosol, scan speed of nozzle and film formation temperature, the film thickness cannot be precisely controlled only by adjusting the film formation time, and the film quality easily changes according to those conditions.
As a related technology, Japanese Patent Application Publication JP-P2001-348659A (page 1 and FIG. 1) discloses an apparatus for fabricating a ceramic structure according to the gas deposition method of spraying an aerosol containing ceramic fine particles on a substrate at a high speed to form a ceramic structure, in which an aerosol containing many primary particles of ceramic in a stable amount over time is generated for adjusting the height of the ceramic structure. In the apparatus for fabricating a ceramic structure, the amount of ceramic fine particles in the aerosol is detected by a sensor, and a signal output from the sensor is fed back to the apparatus for fabricating a ceramic structure.
However, according to JP-P2001-348659A, only the amount of ceramic fine particles in the aerosol, i.e., aerosol concentration is detected by the sensor, but fine particles having different particle diameters and agglomerated particles, which cannot contribute to film formation, contained in the aerosol are not distinguished. Generally, in the case where film formation is performed by employing an aerosol containing many agglomerated particles under the same condition as the normal condition, a structure in a compressed powder state containing many air holes is formed, and thereby, the film quality as represented by density becomes deteriorated. That is, according to the method disclosed in JP-P2001-348659A, the film thickness of the structure (structure height) can be controlled, but the film quality cannot be controlled.
Further, Japanese Patent Application Publication JP-P2002-30421A (page 1 and FIG. 1) discloses a method of forming an ultrafine particle film in an arbitrary film thickness in a gas deposition apparatus, including an ultrafine particle generation chamber provided with an evaporation source and an opening portion of a carrier pipe above the evaporation source and a film formation chamber provided with a nozzle coupled to another opening portion of the carrier pipe and a stage for fixing a substrate provided facing the nozzle thereon, for forming a film by carrying ultrafine particles evaporated from the evaporation source with a gas introduced into the ultrafine particle generation chamber in the carrier pipe and depositing the ultrafine particles injected from the nozzle on the substrate. In the method of producing an ultrafine particle film, the film thickness of the formed ultrafine particle film is measured by a laser film thickness gauge as a contactless film thickness gauge at the same time when an ultrafine particle film is formed on the substrate, and the relative speed between the stage and the nozzle, evaporation source temperature and so on are controlled based on a result of the film thickness measurement.
However, according to the method disclosed in JP-P2002-30421A, it is inevitable that the fine particles, that have been injected from the nozzle but not involved in film formation, adhere to the laser film thickness gauge provided within the chamber, and therefore, the method is unsuitable for film formation for a long period and productivity is low. Further, likewise in JP-P2001-348659A, the film thickness can be controlled but the film quality cannot be confirmed on the moment.
Thus, in JP-P2001-348659A and JP-P2002-30421A, the film quality of the structure cannot be confirmed or controlled. Further, it is still difficult to control the film thickness precisely on the order of micron even by using any one of those methods. For example, in the case where a piezoelectric actuator is fabricated by the AD method, when the film thickness is nonuniform, the applied electric fields vary among plural elements and properties vary, and thereby, the yield in the finished product is reduced. Accordingly, the cost of manufacturing rises. Further, in the case where the structure contains many air holes, this causes reduction in withstand pressure and reduction in density numerically expressed by an elastic modulus and Vickers hardness, and therefore, dielectric breakdown is likely to occur during operation in the finished product.